Guys hoping to find some help please. I am having trouble with the rear caliper pistons, with the brake bled and free from air when I push the pedal down the pistons move slightly but not enough to bite on the disc, when I release the lever they retract slightly.

Is it normal for them to retract slightly? Are the piston seals tapered?

At the brake pedal you will see a rubber boot on the end of the master cylinder.

There is a movable receiver inside that boot for the end of the brake pedal pin. Check that it is properly seated, if not the pedal pushes it to one side and does not force the piston into the master cylinder to operate the brake. Also take the plastic cover off the caliper and check the pads move freely on the pins.

The pads and pistons do retract slightly when you release the pedal.

Check the rear caliper. If you have the original pins in the chances are that they're pretty rusty. If they are, show them to a rotary wire brush in a drill and also consider replacing them with stainless steel pins from Motorworks. Check the pads; they can also rust up and prevent free movement in the caliper. Finally, around that area, scrub all of the dirt out of the caliper as it can build up in the corners and prevent free movement. The only thing is it's a bitch to get off; you need to take the rear wheel off first.

If nothing obvious shows up around the caliper then the most likely source of the problem is the master cylinder. My bet is either that the return hole is blocked or the m/c seals are toast. The easy bit first; make sure the brake pedal has a very small amount free play in it and is not permanently pushing against the rubber. If it's hard up against the rubber block it can push the piston back just far enough to cover the return hole. DAMHIK.

To check the free play you'll need to peel the boot back carefully; if no hydraulic fluid runs out then it's not the seals.

If neither of the above works then there is probably crap in the return hole. The good news is that dismantling the rear m/c is stupidly easy (assuming nothing is corroded up). Dismount it from the footpeg hanger (yep: take the hydraulic hose off). Give it a good clean. On one side is a tiny allen bolt; this holds the piston, spring and seals in. Unscrew the allen bolt and carefully remove the piston assembly. If the seals are intact you don't need an overhaul kit. Clean everything up very carefully and reassemble it with silicon grease (dip it all in hydraulic fluid if you have no silicon grease).

Try bleeding it again.

Edit: as Olaf says about the rubber block. The metal cap on top can also fall off; I glued the my last one back on with Gorilla glue. Then I got really peeved with it (p!ssed off actually) and replaced it with one made from stainless. IMHO it gives a much better feedback from the caliper.

Wow thanks guys for taking the time....gives me hope! I recon I have spent the best part of a day on this and was no further forward than when I started! I should have said at the start that all components are extremely clean,  the MC is actually polished. 

 MC and calipers have all been rebuilt although I have not used new piston seals (as they looked ok) and have not used a MC rebuild kit. The MC was leaking badly until I fitted an EPDM seal behind the top sweeper bushing, now it seems fine.

Now...the allen bolt which holds the MC piston in place the head was damaged so I replaced with a little M5 bolt the same length,  I am now wondering whether that could be making tje piston cover the little return hole? The original bolt has a thinner tip to it.....hmm?

Question, if that return hole is blocked (I did blow it out) would it effect the pressure it could achieve? I will double checknthe pads slide properly on the pins but they have again been wire wheeled and lubricated.

The issue really is lack of pressure. At no point today have I once felt a resistance in the footpedal or a pressure build up, although brake fluid is pumping......

@kustom_moto wrote:MC and calipers have all been rebuilt although I have not used new piston seals (as they looked ok) and have not used a MC rebuild kit. The MC was leaking badly until I fitted an EPDM seal behind the top sweeper bushing, now it seems fine.

This. I bet the fluid is going backwards and forwards around the damaged o-ring seals.

If the o rings in the MC were damaged would it not leak fluid?

If just the front one (the one closest to the hose) is no longer sealing you wouldn't get a leak; you just wouldn't get any pressure. BTDT with a Moto Guzzi rear caliper.

I recon you are bang on actually. Ill be taking the MC apart again later on to take another look......will let know what I find.....

I had the same problem with the master cylinder on my K75RT.  No amount of rebuilding, cleaning, etc. provided any joy.  Trying to make it work was a total waste of time.   I wound up binning it and retrofitting an $11 Chinese master cylinder I got on eBay.  That master cylinder has worked flawlessly now for a full riding season and over 5,000 miles.

http://www.motobrick.com/index.php/topic,9364.25.html

My experience begins at reply #29.

This Chinese MC, is it the one that has links to it at the very start of the thread?

Also, I hear the MC from the K1100 also fits. Will I have the same issue eith this version do you think?

Before you start with a complete panic , strip the m/c and look up inside the body. If there's any sign of the anodising being polished off (shiny silver bits showing through) then it's unlikely to take another overhaul. If it's still all black in there (albeit smoothed out where the o-rings rub) it should take another overhaul.

Problem is is that the overhall kit from Motorworks is £60!!! 

I am still unsure exactly why the piston will not pressurise, the rubber pressure seal looks to be it excellent condition. When I push in the piston and keep my finger over the end hardly any air feels like it wants to escape then when the piston returns I csn feel a definite vacuum pulling my finger into the chamber slightly.

What is the exact issue here? Also, I don't believe this MC chamber was ever black anodised!

I have now ordered the Chinese MC, hopefully this will provide more fruitful!

Been on that journey. Very pleased with my Chinese replacement.